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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Missing Reference: 'RFC5034' is mentioned on line 155, but not defined == Unused Reference: 'RFC1734' is defined on line 267, but no explicit reference was found in the text -- Obsolete informational reference (is this intentional?): RFC 1734 (Obsoleted by RFC 5034) -- Obsolete informational reference (is this intentional?): RFC 2821 (Obsoleted by RFC 5321) -- Obsolete informational reference (is this intentional?): RFC 4234 (Obsoleted by RFC 5234) Summary: 2 errors (**), 0 flaws (~~), 5 warnings (==), 10 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group A. Hathcock 2 Internet-Draft J. Merkel 3 Intended Status: Informational Alt-N Technologies 4 Expires: May 12, 2008 November 12, 2007 6 The Minger Email Address Verification Protocol 7 draft-hathcock-minger-04.txt 9 Status of this Memo 11 By submitting this Internet-Draft, each author represents that any 12 applicable patent or other IPR claims of which he or she is aware 13 have been or will be disclosed, and any of which he or she becomes 14 aware will be disclosed, in accordance with Section 6 of BCP 79. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt. 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 This Internet-Draft will expire on May 12, 2008. 34 Copyright Notice 36 Copyright (C) The IETF Trust (2007). 38 Abstract 40 This document describes the Minger protocol. Minger is a protocol 41 which allows a mail handling entity to query a remote service and 42 ask the question "do you accept mail for this email address?" It 43 includes security in the form of a hashed shared secret but can also 44 be used anonymously if desired. 46 Requirements Language 48 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 49 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 50 document are to be interpreted as described in [RFC2119]. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 55 1.1. The problem . . . . . . . . . . . . . . . . . . . . . . 3 56 1.2. Existing solutions . . . . . . . . . . . . . . . . . . . 3 57 1.3. The Minger solution . . . . . . . . . . . . . . . . . . 4 58 2. The Minger protocol . . . . . . . . . . . . . . . . . . . . 4 59 2.1 The Minger query process . . . . . . . . . . . . . . . . 4 60 2.2 Description of query elements . . . . . . . . . . . . . . 5 61 3. Minger responses . . . . . . . . . . . . . . . . . . . . . . 5 62 3.1 Description of response elements . . . . . . . . . . . . 5 63 3.2 Example responses . . . . . . . . . . . . . . . . . . . . 6 64 4. Anonymous mode . . . . . . . . . . . . . . . . . . . . . . . 6 65 5. Security Considerations . . . . . . . . . . . . . . . . . . 7 66 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 7 67 7. Informative References . . . . . . . . . . . . . . . . . . . 7 68 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . 8 69 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 70 Intellectual Property and Copyright Statements . . . . . . . . . 9 72 1. Introduction 74 1.1 The problem 76 It is common for elements within a typical email handling topology 77 to be unaware of whether individual local-parts are valid for the 78 mail it accepts. For example, so-called "edge" servers which provide 79 security oriented services for downstream mail handling elements 80 often do not have an exhaustive listing of all valid local-parts for 81 a given domain. Thus, they are sometimes forced to accept and process 82 messages which might otherwise be rejected as "user unknown". 83 Similarly, entities offering "backup MX" mail services are rarely 84 privy to a complete local-part listing and are therefore often decide 85 to accept messages which might otherwise be rejected. Finally, even 86 within a common administrative framework of several locally maintained 87 and controlled SMTP servers in a load balanced configuration, it is 88 not always possible for all servers to access a common local-part 89 database. 91 1.2 Existing solutions 93 The need to determine whether an email address contains a valid local 94 part has lead to the use of at least two existing mechanisms - Finger 95 [RFC1288] and SMTP "call-forward". 97 Finger [RFC1288] describes a protocol for the exchange of user 98 information. In theory, Finger could be used to determine whether an 99 account exists by careful examination of the results of a Finger 100 query. However, Finger suffers from a lack of security which makes 101 its modern day use problematic when coupled with the user level 102 detail it can provide. Also, Finger requires the use of TCP rather 103 than UDP which seems ill suited to a simple verification scheme. 105 SMTP "call-forward" is a term used to describe a widespread practice 106 whereby SMTP servers place an incoming SMTP session on hold while they 107 attempt to use an outbound SMTP session to determine whether or not a 108 given email address is valid. The theory behind this is as follows: 109 if an SMTP server responds positively to an SMTP RCPT command 110 [RFC2821] with a given email address then this potentially means that 111 the address local-part is valid. One problem with such a scheme is 112 the lack of efficiency inherent in the need to tear-up and tear-down 113 an SMTP session over TCP. Also, because these types of SMTP sessions 114 are not purposed to deliver mail, they typically drop connection after 115 the RCPT command is processed. This leads to a large number of SMTP 116 sessions which appear in logs to have simply failed for no reason. 117 This leads to situations in which SMTP transaction logs can no longer 118 distinguish legitimate network errors from "call-forward" traffic. 120 SMTP includes a VRFY command which can be used to determine whether 121 an email address exists. VRFY is not in wide-spread use and suffers 122 from the same inefficiency concerns described in the discussion on 123 SMTP "call-forward". Additionally, SMTP agents providing mail 124 services to a domain are often not authoritative making VRFY requests 125 potentially unreliable. 127 LDAP could be used to determine whether an email address exists. 128 However, LDAP is overly complex to configure and maintain. 130 1.3 The Minger solution 132 What's needed is a protocol which is secure, has little overhead, and 133 can be easily invoked to determine whether a given email address is 134 valid or not. Minger achieves these goals using a shared secret for 135 security and UDP to lower overhead. 137 2. The Minger protocol 139 Minger is a UDP protocol that operates on port 4069. 141 Syntax descriptions use the form described in Augmented Backus-Naur 142 Form for Syntax Specifications (ABNF) [RFC4234]. 144 2.1 The Minger query process 146 A Minger client constructs a query string as described below and 147 transmits it over UDP to a Minger server. The format of the query 148 is as follows: 150 ABNF: 152 query-string = mailbox [SP "d=" digest] [SP tag-list] 154 digest = base64 ; digest for security 155 ; base64 defined in [RFC5034] 157 digest-text = shared-secret ":" mailbox ; input text for digest 159 mailbox = Local-part "@" Domain ; as defined in [RFC2821] 161 shared-secret = 1*50(VCHAR) ; password credential 162 tag-list = tag-spec *(SP tag-spec) ; tag/value list 164 tag-spec = tag-name "=" tag-value 166 tag-name = 1 * ( ALPHA / DIGIT / "_") ; except 'd' 168 tag-value = 1 * (ALPHA / DIGIT / "_") 170 2.2 Description of query elements 172 mailbox 174 This is the email address for which verification of 175 existence is desired. 177 digest 179 This is the base64 encoding of the MD5 [RFC1321] hash of 180 digest-text. Digest-text is constructed, the MD5 hash of that 181 is computed, and that result is base64 encoded. 183 tag-list 185 Tag-list is provided so that future capability might be added 186 in an easy way. Tag-names are case-sensitive and MUST NOT 187 be used more than once. 189 3. Minger responses 191 Minger servers return a response string of the following form: 193 ABNF: 195 response-string = mailbox status 197 mailbox = Local-part "@" Domain ; as defined in [RFC2821] 199 status = %x30-35 ; single digit result code 200 ; from 0 - 5 202 3.1 Description of response elements 204 mailbox 206 This is the email address for which verification of 207 existence is desired. 209 status 211 The following status codes are defined: 213 0 - invalid request (for example, malformed query string) 214 1 - access denied (for example, query from unauthorized IP) 215 2 - bad or missing credentials (returned when anonymous 216 mode is disabled and no credentials were provided in the 217 query string or when the credentials themselves are 218 invalid) 219 3 - email address does not exist 220 4 - email address exists but can not receive mail (for example, 221 the account associated with the email address has exceeded 222 local storage constraints or it is otherwise disabled due 223 to local policy) 224 5 - email address exists and is active (able to receive mail) 226 3.2 Example responses 228 Minger response returned when the queried email address does 229 not exist: 231 arvel@example.com 3 233 Minger response returned for invalid credentials: 235 arvel@example.com 2 237 Minger response returned when the queried email address exists: 239 arvel@example.com 5 241 4. Anonymous mode 243 Minger clients MAY attempt anonymous queries; that is, queries which 244 do not contain a shared secret digest within the query string. Minger 245 servers MAY be configured to refuse anonymous queries. If so, they 246 MUST respond with a status of "2". 248 5. Security Considerations 250 Minger is a protocol which is used to determine whether a given 251 email address is valid or not. If a particular email 252 infrastructure does not wish to advertise the email addresses that 253 it services then this protocol should not be employed. 255 If a shared secret is employed to secure Minger from anonymous use 256 that shared secret should be at least 128 bits. 258 6. IANA Considerations 260 IANA has assigned tcp & upd port 4069 for Minger. 262 7. Informative References 264 [RFC1288] Zimmerman, D., "The Finger User Information Protocol", 265 RFC 1288, December 1991. 267 [RFC1734] Myers, J., "POP3 Authentication Command", RFC 1734, 268 December 1994. 270 [RFC2821] Klensin, J., Editor, "Simple Mail Transfer Protocol", RFC 271 2821, March 2001. 273 [RFC4234] Crocker, D., Ed. And P. Overell, "Augmented BNF for Syntax 274 Specifications: ABNF", RFC 4234, October 2005. 276 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 277 Requirement Levels", BCP 14, RFC 2119, March 1997. 279 [RFC1321] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, 280 MIT Laboratory for Computer Science and RSA Data Security, 281 Inc., April 1992. 283 Appendix A. Acknowledgements 285 We wish to thank the members of the MDaemon Beta Community 286 (md-beta-subscribe@altn.com) for their ideas and help and Paul 287 Hoffman for his valuable feedback. 289 Authors' Addresses 291 Arvel Hathcock 292 Alt-N Technologies 293 http://www.altn.com 295 Email: arvel.hathcock@altn.com 297 Jonathan Merkel 298 Alt-N Technologies 299 http://www.altn.com 301 Email: jon.merkel@altn.com 303 Full Copyright Statement 305 Copyright (C) The IETF Trust (2007). 307 This document is subject to the rights, licenses and restrictions 308 contained in BCP 78, and except as set forth therein, the authors 309 retain all their rights. 311 This document and the information contained herein are provided on an 312 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 313 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 314 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 315 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 316 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 317 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 319 Intellectual Property 321 The IETF takes no position regarding the validity or scope of any 322 Intellectual Property Rights or other rights that might be claimed to 323 pertain to the implementation or use of the technology described in 324 this document or the extent to which any license under such rights 325 might or might not be available; nor does it represent that it has 326 made any independent effort to identify any such rights. 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